SUMMARY

Hagfish slime consists of mucins and protein threads that are released from
slime glands and mix with seawater to produce an ephemeral material with
intriguing physical properties. We recently characterized the mechanics of the
slime's fibrous component, and here we report the first mechanical properties
of the mucin component and the slime as a whole. Our results suggest that
hagfishes can produce remarkable quantities of the slime because it is almost
three orders of magnitude more dilute than typical mucus secretions.
Mechanical experiments using whole slime produced in vitro
demonstrate that the slime threads dominate the slime's material properties
and impart elasticity. Mucins impart viscosity at the strain rates tested and
are important for rapid deployment of the slime. We also found that slime
threads are tapered at both ends, which suggested to us that hagfish slime
might best be modeled as a discontinuous fibre-reinforced composite. Our
measurements demonstrate that the mucins are not capable of providing shear
linkage between threads, but this is not necessary because the threads are
long enough to span an entire slime mass. Our findings suggest that hagfish
slime consists mainly of bulk seawater entrained between mucin-coated threads,
and in this way functions more like a fine sieve than coherent mucus. These
results are consistent with the hypothesis that the slime has evolved as a
defense against gill-breathing predators.